Unbreak Finalize HashAggregate over Partial HashAggregate.

Commit 5dfc198146b49ce7ecc8a1fc9d5e171fb75f6ba5 introduced the use
of a new type of hash table with linear reprobing for hash aggregates.
Such a hash table behaves very poorly if keys are inserted in hash
order, which does in fact happen in the case where a query use a
Finalize HashAggregate node fed (via Gather) by a Partial
HashAggregate node.  In fact, queries with this type of plan tend
to run effectively forever.

Fix that by seeding the hash value differently in each worker
(and in the leader, if it participates).

Andres Freund and Robert Haas

1 files changed, 18 insertions, 2 deletions

diff --git a/src/backend/executor/execGrouping.c b/src/backend/executor/execGrouping.c
index 94cc59de7e1..3149fbe975c 100644
--- a/src/backend/executor/execGrouping.c
+++ b/src/backend/executor/execGrouping.c
@@ -18,6 +18,8 @@
  */
 #include "postgres.h"
 
+#include "access/hash.h"
+#include "access/parallel.h"
 #include "executor/executor.h"
 #include "miscadmin.h"
 #include "utils/lsyscache.h"
@@ -289,7 +291,8 @@ BuildTupleHashTable(int numCols, AttrNumber *keyColIdx,
 					FmgrInfo *eqfunctions,
 					FmgrInfo *hashfunctions,
 					long nbuckets, Size additionalsize,
-					MemoryContext tablecxt, MemoryContext tempcxt)
+					MemoryContext tablecxt, MemoryContext tempcxt,
+					bool use_variable_hash_iv)
 {
 	TupleHashTable hashtable;
 	Size		entrysize = sizeof(TupleHashEntryData) + additionalsize;
@@ -314,6 +317,19 @@ BuildTupleHashTable(int numCols, AttrNumber *keyColIdx,
 	hashtable->in_hash_funcs = NULL;
 	hashtable->cur_eq_funcs = NULL;
 
+	/*
+	 * If parallelism is in use, even if the master backend is performing the
+	 * scan itself, we don't want to create the hashtable exactly the same way
+	 * in all workers. As hashtables are iterated over in keyspace-order,
+	 * doing so in all processes in the same way is likely to lead to
+	 * "unbalanced" hashtables when the table size initially is
+	 * underestimated.
+	 */
+	if (use_variable_hash_iv)
+		hashtable->hash_iv = hash_uint32(ParallelWorkerNumber);
+	else
+		hashtable->hash_iv = 0;
+
 	hashtable->hashtab = tuplehash_create(tablecxt, nbuckets);
 	hashtable->hashtab->private_data = hashtable;
 
@@ -450,7 +466,7 @@ TupleHashTableHash(struct tuplehash_hash *tb, const MinimalTuple tuple)
 	TupleHashTable hashtable = (TupleHashTable) tb->private_data;
 	int			numCols = hashtable->numCols;
 	AttrNumber *keyColIdx = hashtable->keyColIdx;
-	uint32		hashkey = 0;
+	uint32		hashkey = hashtable->hash_iv;
 	TupleTableSlot *slot;
 	FmgrInfo   *hashfunctions;
 	int			i;